1. Field of the Invention
The present invention relates to protection diodes and more particularly to low voltage protection diodes.
2. Discussion of the Related Art
A unidirectional protection diode Z1 (FIG. 1A) is a component allowing current to flow in a forward direction (from terminal B to terminal A) and blocking current in the reverse direction (from terminal A to terminal B) as long as the voltage does not exceed a predetermined threshold. A bidirectional diode Z2 (FIG. 2A) is a component allowing current to flow from terminal C to terminal D or from terminal D to terminal C only when the voltage between these terminals exceeds a predetermined threshold.
Such diodes are disposed in parallel on circuits to be protected and are operative only when exceptional voltage surges occur. Therefore, during a normal state of the power supply or under normal operating conditions of the circuit, the diodes must not impair the circuit operation. However, it is known that a low voltage protection diode has a relatively high capacitance between its terminals. Indeed, the capacitance across a diode increases in proportion to the surface area of the useful junction of this diode and exponentially with the doping level of the least doped layer at the junction. In practice, in the case of a protection diode, if it is desired to obtain a relatively low breakdown voltage, ranging approximately from 5 to 10 volts, the doping level of the least doped layer must be high (approximately 1 to 5.times.10.sup.18 atoms/cm.sup.3) and, therefore, the diode capacitance is unavoidably high. For example, a low voltage protection diode having a breakdown voltage of approximately 5 volts has a capacitance of approximately 5000 pF/mm.sup.2. To meet its protection function requirements, the surface area of this type of component is seldom lower than 0.5 mm.sup.2 ; this leads to a 2500 pF capacitance. This relatively high value is liable to impair the operation of the circuit to be protected.
To solve this problem, it has been proposed in the prior art to combine the protection diodes with simple conventional rectifying diodes in various circuit configurations. All these various configurations have exhibited drawbacks and have not enabled hitherto realization of monolithic protection components.